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Several standardized ignition sources are used to determine the safety characteristics of gases, vapours and dusts. Standards indicate the source of ignition but vary in specifying other features such as the burning duration, energy or the volume in which the energy is released. Since heat is not visible under normal conditions, a schlieren technique was used to visualize the entire igniting volume and not just the flames. This article focuses on the igniting volume, compares it among the four standardized ignition sources and displays its relationship to the size of the test vessel. Differences in the ignition behaviour of the ignition sources might lead to the determination of erroneous safety characteristics and with that to the unsafe operation of processes.
The hybrid mixture of combustible dusts and flammable gases/vapours widely exist in various industries, including mining, petrochemical, metallurgical, textile and pharmaceutical. It may pose a higher explosion risk than gas/vapor or dust/mist explosions since the hybrid explosions can still be initiated even though both the gas and the dust concentration are lower than their lower explosion limit (LEL) values. Understanding the explosion threat of hybrid mixtures not only contributes to the inherent safety and sustainability of industrial process design, but promotes the efficiency of loss prevention and mitigation. To date, however, there is no test standard with reliable explosion criteria available to determine the safety parameters of all types of hybrid mixture explosions, nor the flame propagation and quenching mechanism or theoretical explanation behind these parameters. This review presents a state-of-the-art overview of the comprehensive understanding of hybrid mixture explosions mainly in an experimental study level; thereby, the main limitations and challenges to be faced are explored. The discussed main contents include the experimental measurement for the safety parameters of hybrid mixtures (i.e., explosion sensitivity and severity parameters) via typical test apparatuses, explosion regime and criterion of hybrid mixtures, the detailed flame propagation/quenching characteristics behind the explosion severities/sensitivities of hybrid mixtures. This work aims to summarize the essential basics of experimental studies, and to provide the perspectives based on the current research gaps to understand the explosion hazards of hybrid mixtures in-depth.
Influence of pre-ignition pressure rise on safety characteristics of dusts and hybrid mixtures
(2021)
For the determination of the safety characteristics of dusts it is necessary to disperse the dust in the oxidating atmosphere (usually air). In the standard procedures for dusts this is realized by a partially evacuated explosion vessel (20L-sphere) in which the dust gets injected from a dust chamber pressurized with air. Shortly after that injection (60 ms) the dust cloud gets ignited under turbulent conditions, that are otherwise seen as almost ambient with 20 ◦C and about 1 bar (abs). While there has been a lot of research about the influence of the ignition delay time and the level of turbulence in the recent years little attention was paid to the pre–ignition pressure rise and the allowed variations in the standards. In the following work we showed that the allowed ranges for the pressures in the different dust standards influence the safety characteristics of dust alone severely.
Even though hybrid mixtures are an emerging risk problem in an interconnected industry there is no standard for the determination of their safety characteristics. In this work it is shown that especially for the preparation of hybrid mixtures of flammable dust and gas the pressures after injection of the dust and the mixing procedure have a large influence on the composition of the tested mixtures and therefore on the safety characteristics.
Considering both effects, wrong concentration of gas and wrong initial pressure, the discrepancy of safety characteristics from different facilities will be too big to applicable. The methods to overcome these weaknesses are also presented.
In the standards for the determination of safety characteristics of dusts an initial pressure difference of 400 mbar (all: abs) is applied, that increases to 1013 mbar to achieve a homogeneous dust-air mixture. Since the measuring equipment accuracy is given with ± 0,1 bar the initial pressure could range somewhere between 300 and 500 mbar and the pressure increase between 500 and 700 mbar, leading to a testing pressure of 800 to 1200 mbar.
This variation does not only affect the testing pressure but also the concentration of dust and the level of turbulence, leading to wrong values of Pmax, Kst and assigning them to wrong concentrations. When tests for hybrid mixtures are conducted there are more degrees of freedom leading to even higher variations. To overcome these variations a higher accuracy is demanded for hybrid mixture testing. Further, in this presentation four standardized ignition sources are presented.
Voraussetzung zur Aufklärung von Fugenbewegungen in hochbeanspruchten Verkehrsflächen aus Beton ist ein neues, innovatives und sensitives Sensorsystem, welches unter den Bedingungen der Autobahnpraxis schnell und sicher in entsprechend beanspruchte Bereiche installiert werden kann und in der Lage ist, stabile und hochaufgelöste Bewegungen in mehrere Raumrichtungen zu erfassen. Das durch die BAM neu entwickelte Sensorsystem ist geeignet, um direkt in die Betonfahrbahndecke integriert zu werden und sowohl über saisonale Messbereiche als auch in hoher Auflösung entsprechende Messwerte online zu erfassen und bereit zu stellen. Das für diesen Zweck entwickelte innovative Sensorsystem kann direkt in die Rollspur auf beiden Seiten der Fuge eingebaut werden und ist dafür ausgelegt, Lkw-Überfahrungen zu widerstehen. Es ist schnell und präzise genug, um die realen Bewegungen in allen drei Raumachsen in Echtzeit erfassen zu können. Dieser Forschungsbericht beschreibt das Funktions- und Wirkschema des Sensorsystems und seine Validierung im Labor- und Feldmaßstab. Dabei wird insbesondere auch die praxisgerechte Einbau- und Nutzungsmethodik vorgestellt. Auflösungsvermögen, Robustheit und Nutzerfreundlichkeit werden am Beispiel einer Konzeptstudie auf dem Testgelände DuraBASt erprobt. Es werden grundlegende Hinweise auf den dringlichen Bedarf einer gebrauchsgerechten Beschreibung des realen Verhaltens des Bauwerks Betonstraße identifiziert.
Die mit dem Sensorsystem gewonnenen Daten können eine Grundlage für die Konzeption einer performance-basierten Bewertung von Fugenfüllsystemen in Betondecken von Bundesautobahnen bieten. Sie sind geeignet, die Funktionsmechanismen der verschiedenen Betonfahrbahnkonstruktionen besser zu verstehen und zielgerichtet konstruktive und materialtechnische Optimierungen und Fortentwicklungen von Fugenkonstruktionen und Fugenfüllsystemen in gebrauchsbezogener Weise zu entwickeln. Durch weitere Datenerhebung, -fusion und -analyse können Instandsetzungsintervalle und Lebensdauerzyklen besser abgeschätzt und geplant werden.
Die zielgerichtete Weiterentwicklung von Bauteilen und Konstruktionselementen im Straßenbau hochbeanspruchter Verkehrswege unserer Infrastruktur (Bundesfernstraßen) erfordert spezielle, bauteiladaptierte technische Möglichkeiten/Sensorik zur Quantifizierung des Gebrauchsverhaltens. Bei Fugen in Verkehrsflächen stellen insbesondere langsam und schnell ablaufende Plattendeformationen infolge jahreszeitlicher und verkehrlicher Beanspruchungen maßgebende Beanspruchungszustände dar. Eine Quantifizierung dieser Einwirkungen hilft bei der Weiterentwicklung sowie auch bei der Bewertung optimierter technischer Lösungen. In Forschung kompakt 17/21 „Innovative Sensorik für Fugensysteme“ wird eine neuartige, robuste Lösung der BAM zur Datenerfassung und Bauwerksmonitoring von hochbeanspruchten Verkehrsflächen aus Beton vorgestellt.
SENSO JOINT - An innovative sensor system for a sustainable joint design of concrete pavements
(2020)
Inacceptable capability and durability of joint sealing systems but also inadequate traffic performance (noise emission; overrolling comfort) up to traffic safety aspects reflect the still enormous demand for data-based description of concrete pavements performance under heavy loading conditions. Especially the deformation behavior of concrete pavement slabs in the joint region in consideration of new pavement construction types and improved concrete mixtures meanwhile established but also under the steeply rising traffic loads is not sufficiently explored. To create a data basis for advanced design rules, evaluation methods and product standards - and with it to improve quality, durability and finally sustainability of pavements - an innovative 3-D sensor system SENSO JOINT adapted to german roadworking requirements and suitable for heavy-duty operating conditions was developed. The contribution introduced describes the development of an extensive technical solution based on the analysis of decisive loads, interactions and boundary conditions. Based on calibration data, results of laboratory testing and finally field-testing on different concrete pavement construction types the outcome of a multi-level evaluation process shall introduce the potential of the new sensor system.